1. Field of the Invention
The invention relates generally to circuits to be used in fault tolerant computing and more particularly to a voting circuit whereby multiple computational channels are polled to provide a highly reliable output.
2. Description of the Prior Art
A computer will behave in a specified manner as long as the parameters of physical components and the speed of operation remain within specified limits. However, it has been a common experience that unexpected abnormal physical changes in component parameters do occur in all kinds of computers. They are usually called malfunctions when the changes are temporary, and failures when the changes are permanent. Their effect is to cause an unspecified and disruptive change in one or more logic variables of the computer. Such a change is called a physical fault, or simply a fault when the physical nature of the fault is clear. Non-physical faults are referred to as "man-made" faults.
The possibility of randomly occuring faults makes the user uncomfortably aware of the physical side of his computer. A fault in a computer on-board a planetary spacecraft can mean loss of a mission. In commercial jets computers are used for functions such as navigation, stability augmentation, flight control and system monitoring. While performance of these functions by a computer is not critical, a fault may require significant disruption such as a change in destination. The usual solution to the problem of a failure is to manually remove and repair the cause of the fault.
The purpose of fault-tolerance is to offer an alternate solution to the fault problem in which the detection of faults and the recovery to normal operation are carried out as internal functions of the system itself. A number of articles dealing with various aspects of fault-tolerant digital systems appear in Proceedings of the IEEE, Volume 66, Number 10, October 1978.
In the prior art the principles of fault-tolerance have been limited in application to the internal functions of the system itself. However, peripheral devices themselves may experience faults which can render the entire system inoperable regardless of the computers fault-tolerant features. Most computer systems use external memory storage such as floppy disks for storing the operating system program, the compiler and other system programs. Without reliable transfer to and from external devices the overall system will be incapable of fault-tolerant operation.